This invention relates to tire inflation and evacuation methods and apparatus.
It is widely accepted that deterioration of rubber-based vehicle tires is attributable, at least in part, to the presence of oxygen in the gas with which the tires are inflated. For this reason it is common practice, for instance in the case of aircraft tires, to inflate the tires with inert nitrogen rather than compressed air. Conventionally, this is achieved by inflating the tires from cylinders containing compressed nitrogen.
If a tire previously inflated with air is to be inflated with nitrogen, the tire valve is initially removed or manipulated to allow the air to escape. While the air pressure in the tire is greater than ambient atmospheric pressure, the air will escape freely. However, when the internal and external pressures reach equilibrium, the tire will remain charged with a volume of air at atmospheric pressure. In an attempt to get rid of this residual air, the existing practice is to use a purging system in which, after deflation of the tire to atmospheric pressure, the tire is re-inflated with pure nitrogen. The oxygen content of the residual air is accordingly diluted. The tire is once again deflated and immediately re-inflated once again with pure nitrogen, thereby further diluting the oxygen content of the gas within the tire. This process of deflation and re-inflation may have to be carried out several times to achieve an acceptably pure nitrogen level. Repetitive deflation and re-inflation is, however, time-consuming and wasteful of nitrogen.
It has been unexpectedly discovered that when there is less than about 5% (by volume) O.sub.2, and preferably 3% (by volume) O.sub.2 in the gas mixture filling the tire, deterioration of the tire is substantially similar to deterioration of a tire filled with substantially pure nitrogen, while deterioration of the tire substantially increases when the tire is filled with air. Maximum deterioration of the tire occurs when the tire is filled up with air, i.e., with a gas mixture comprising about 21% (by volume) O.sub.2.